'And do people usually store their cels by size or by series
                  or some other way?'

I store by size, although if I have alot of cels from one 
series which will fill up a couple of folders by themselves, 
I will just label it "Esca cel book 1 (or 2, or whatever)" then stuff it 
to capacity :)
 I need to find cels quickly so it helps to store by series...  

Oversize cels I have fewer of, so I store them by size in an 
A3 size folder from Sepal. All my other folders are now B4 size... 
more convenient since backgrounds sometimes are wider than the
cel alone.

Speaking of brand names and Kinokuniya, I am surprised no one 
has mentioned any of the other brands like Nakabayashi, Sepal,
Data Note (I think thats right .. ^^), etc ...  cheap Sepal 
folders can be purchased for less than $10 (for the transparent cover ones,
which are wider - good for backgrounds and pan cels -  and 
stiffer than the small Kokuyo folders I have seen)
All of the above brands I have purchased at my local Kinokuniya. 
So of course, these are import prices. 

Humongous cels probably would need the folders made for posters,
which I have not seen at Kinokuniya but they are available at
other stores here. (here being s.e.asia)

As for the acid free question, I don't know if any of the above 
are or not ^^ They are just convenient so I use them.

-Ash